The invention relates to a method of determining the position k of the tap on a resistance teletransmitter comprising two fixed resistance ranges, between which an effective resistance range is provided, in which the tap is adjustable. The first connection of the resistance teletransmitter is connected to a current source and can be connected through a first conductor to an instrument amplifier, its tap constituting a central connection is connected to a second conductor and its second connection can be connected through a third conductor to the instrument amplifier.
Such a method and a circuit arrangement for carrying out this method are known from Philips apparatuses KS 4400 or KS 4450. Such apparatuses serve, for example, for recording the height of the level of a liquid. Two current sources are then utilized, which are each connected to an input of an instrument amplifier. One current source is connected to the first connection of the resistance teletransmitter and the other current source is connected to the second connection of the resistance teletransmitter. The adjustable tap on the resistance teletransmitter constituting the central connection is connected to zero potential. Such a resistance teletransmitter has an effective resistance range, in which the tap is adjustable, and two fixed resistance ranges, which are each applied to the first and second connection, respectively, of the resistance teletransmitter. The two fixed resistance ranges and the effective resistance range can be adjusted by the user, that is to say that the resistance values of the fixed resistance ranges may be equal to zero. The output of the instrument amplifier is connected to the input of a succeeding analogue-to-digital converter, which supplies at its output digital signals for an indication device or for a control unit.
A disadvantage of this known method and known circuit arrangement is that the current sources must be balanced to equality in order that the measurement becomes independent of the conduction resistances of the first, second and third conductors. Likewise, amplification errors and the offset of the instrumental amplifier must be compensated for, which involves complicated operations. Since the measured values are represented in a digital indication device, in the known circuit arrangement an analogue-to-digital converter is utilized. As a result, besides the complicated balancing step for eliminating the conduction resistances, the temperature drift of the instrumental amplifier and in addition the temperature drift of the analogue-to-digital converter also influence the measuring result.
The known method and the known circuit arrangement are therefore not suitable for use in devices which from a commercial view-point must lie at a low price level (low-cost device) because analogue-to-digital converters having a very small temperature drift represent a considerable factor of cost.
It should be noted that Kokai 52-145 246 discloses a circuit arrangement for determining the resistance value of an effective resistance range by means of a resistance teletransmitter. In this case, the first connection of the resistance teletransmitter is connected through a first conductor to a current source, the central connection is connected through a second conductor to the non-inverting input of an amplifier and the second connection is connected on the one hand through a third conductor to the inverting input of the amplifier and on the other hand through a fourth conductor to ground. In this arrangement, elimination of the conduction resistances is also attained. However, only one resistance range is determined. The tap position of the resistance teletransmitter cannot be determined without further expedients. Moreover, this circuit arrangement is complicated and expensive due to the use of a fourth conductor.
DE OS No. 3101994 discloses a circuit arrangement, which relates to the measurement of an electrical resistance. In this circuit arrangement, three switches each having two closing contacts are present. The first closing contact of the first switch connects a current source to an auxiliary resistor connected to ground; the other closing contact is coupled to an analog-to-digital converter. The second switch connects the current source, the analog-to-digital converter and a reference resistor to each other. The reference resistor is connected to the ground-free connection of the auxiliary resistor. The third switch connects the resistor to be measured, which is also connected to the auxiliary resistor, to the current source and to the analog-to-digital converter. Of the three switches, which are successively actuated, only one is closed at a time. A computer succeeding the analog-to-digital converter calculates from the three voltages measured at the analog-to-digital converter the resistance of the resistor to be measured. The measuring values do not exhibit temperature-dependent errors. With this circuit arrangement it is not possible to determine the position k of a resistance teletransmitter. Moreover, a compensation of conduction resistances cannot be obtained by this measuring arrangement.